While books are important, there's nothing like good ol' intuition. Brian is hinting at it. There are four things to remember:
A) The system worked when it was new (supposedly). Something has changed at some point; you need to figure out what it is.
2) Troubleshooting is a process of elimination. Deduce the possible problems by noting the symptoms, and rule them out, easiest-to-check first.
D) Don't forget to start with the basics: try bulbs in a good lamp, make sure the plugs are plugged in, test for power at breaker terminals, etc.